Rare earth sintered magnets, such as R-T-B-based sintered magnets (R means at least one of rare earth elements (concept including yttrium (Y)), T means iron (Fe) or a combination of iron and cobalt (Co), and B means boron) and Sm—Co-based sintered magnets (samarium (Sm) may be partially substituted with other rare earth elements) are widely used because of excellent magnetic characteristics such as a residual magnetic flux density Br (hereinafter sometimes simply referred to as “Br”) and a coercive force Hcj (hereinafter sometimes simply referred to as “Hcj”).
Particularly, the R-T-B based sintered magnet has the highest magnetic energy product among various magnets hitherto known, and is relatively inexpensive. Thus, the R-T-B based sintered magnet has been used for various applications, including various motors, such as a voice coil motor for a hard disc drive, a motor for a hybrid vehicle, and a motor for an electric vehicle, and home electric appliances. In recent years, in order to achieve the reduction in size and weight or higher efficiency of products for various applications, the rare earth sintered magnets, such as the R-T-B based sintered magnet, are required to further improve its magnetic characteristics.
The production of most of the rare earth sintered magnets including an R-T-B-based sintered magnet includes the following steps of:
obtaining a raw material alloy cast with a desired composition, such as an ingot produced by melting (fusing) raw materials for examples metals and casting the molten raw materials in a die to obtain an ingot, or strip produced by a strip cast method, and grinding the raw material alloy cast to produce alloy powder having a predetermined particle diameter; and
press molding the alloy powder (press molding the alloy powder in a magnetic field) to produce a molded body (green compact), and then sintering the molded body.
In the case of obtaining an alloy powder from a casting material, in many cases, steps to be used are two grinding steps of a coarsely grinding step of grinding into a coarse powder having a large particle diameter (coarsely ground powder) and a finely grinding step of further grinding the coarse powder into an alloy powder having a desired particle diameter.
The method of press molding (press molding in a magnetic field) is roughly classified into two methods. One is a dry molding method in which the obtained alloy powder is subjected to press molding in a dry state. The other one is a wet molding method mentioned, for example, in Patent Document 1, in which an alloy powder is dispersed in a dispersion medium such as oil to prepare a slurry, and the alloy powder is supplied in a cavity of a die in a state of the slurry, followed by press molding.
Furthermore, the dry molding method and the wet molding method can be roughly classified into two methods, respectively, according to a relation between the pressing direction at the time of pressing in a magnetic field and the direction of the magnetic field. One is a perpendicular magnetic field molding method (also referred to as a “transverse magnetic field molding method”) in which the direction of compression performed by a press (pressing direction) is orthogonal to the direction of the magnetic field applied to an alloy powder. The other one is a parallel magnetic field molding method in which the pressing direction is in parallel with the direction of a magnetic field applied to an alloy powder (also referred to as a “longitudinal magnetic field molding method”).
There is a need for the wet molding method to perform supply of a slurry and removal of a dispersion medium, and thus the structure of a molding device becomes comparatively complicated. However, oxidation of the alloy powder and the molded body is suppressed by the dispersion medium, thus enabling reduction in the amount of oxygen of the molded body. The dispersion medium exists between alloy powders at the time of press molding in the magnetic field, leading to weak restriction due to a friction force. Thus, the alloy powder can rotates more easily in the magnetic field application direction. Therefore, higher orientation degree can be obtained. Thus, it is possible to obtain a rare earth sintered magnet which is more excellent in magnetic characteristics as compared with the dry molding method.
High orientation degree and excellent oxidation suppressing effect obtained using the wet molding method can be obtained in not only this R-T-B-based sintered magnet, but also other rare earth sintered magnets.
Among the wet molding methods, especially, the use of the parallel magnetic field molding method can achieve the more excellent magnetic characteristics based on the following reasons.
In the wet molding method, when the slurry is charged in a cavity and press molding is performed in the magnetic field, there is a need for most of a dispersion medium (oil, etc.) in the slurry to be discharge out of the cavity. Usually, at least one of an upper punch and a lower punch is provided with a dispersion medium outlet and, when the volume of the cavity decreases by the movement of the upper punch and/or the lower punch to pressurize the slurry, the dispersion medium is discharged through the dispersion medium outlet. In this case, since the dispersion medium in the slurry is filtered and discharged from the portion close to the dispersion medium outlet, a layer called a “cake layer” having increased concentration (high density) of the alloy powder is formed at the portion close to the dispersion medium outlet in an initial stage of press molding.
As the upper punch and/or the lower punch move (s) and press molding proceeds, much more dispersion medium is filtered and discharged, and thus an area of the cake layer spreads in the cavity. Finally, the cake layer having high density of the alloy powder (low dispersion medium concentration) spreads all over the cavity, resulting in achieving bonding between the alloy powders (comparatively weak bonding) to obtain a molded body.
In the initial stage of press molding, when the cake layer is formed at the portion close to the dispersion medium outlet (upper portion and/or lower portion in the cavity), the direction of the magnetic field tends to be curved in the perpendicular magnetic field molding method.
The cake layer exhibits increased magnetic permeability as compared with the portion other than the cake layer of the slurry (portion with less amount of the alloy powder per unit volume) because of high density of the alloy powder (large amount of the alloy powder per unit volume), thus causing focusing of the magnetic field in the cake layer. This means the fact that, even if the magnetic field is applied approximately perpendicularly to the cavity side surface outside the cavity, the magnetic field is curved to the cake layer inside the cavity. Therefore, since the alloy powder is oriented along this curved magnetic field, the portion with curved orientation exists in the molded body after press molding, leading to a decrease in orientation degree in the single molded body, thus failing to obtain sufficient magnetic characteristics in the sintered magnet.
Meanwhile, in the parallel magnetic field molding method, since the magnetic field is applied to the direction parallel to the pressing direction, i.e. the direction parallel to the direction from the upper punch toward the lower punch, even if the cake layer is formed at the portion close to the dispersion medium outlet of the upper punch and/or the lower punch, the magnetic field travels straight toward the inside of the cake layer from the portion where the cake layer does not exist without being curved. Therefore, this does not cause the bending of the orientation of the magnetic field, unlike the perpendicular magnetic molding method.    Patent Document 1: JP 8-69908 A